Raman spectrometry has an advantage that it can be performed regardless of whether the sample is a gas, a liquid, a crystal, or an amorphous solid, and regardless whether the temperature is high or low. Further, Raman spectrometry does not require any special measurement atmosphere such as a vacuum in the measurement. Further, Raman spectrometry has another advantage that it does not require any particular preprocessing of the sample and thus the sample can be measured as it is. Therefore, a number of measurements have been carried out by taking these advantages. By using Raman spectrometry, it is possible to observe molecules without dyeing them and to observe impurities in a semiconductor.
In order to perform such Raman spectrometry, Raman microscopes using spectroscopes have been disclosed (Patent literatures 1 and 2), in FIG. 1 of Patent literature 1, Raman-scattered light generated in a sample is divided into a spectrum by a spectroscope. Further, an entrance slit is disposed in front of the spectroscope. Then, Raman-scattered light generated in a certain point of the sample passes through a spot-like area of the entrance slit. An optical microscope disclosed in Patent literature 2 also includes an entrance slit. In an ideal case where no aberration occurs in the spectroscope, light that is generated in a certain point of the sample and passes through the spot-like area is dispersed in a λ-direction and detected in one pixel row of the detector. In such a case, Raman-scattered light having a specific wavelength generated in a certain point of the sample forms a light beam spot 81 on the light-receiving surface of the detector as shown in FIG. 13. FIG. 13 shows Raman-scattered light having two specific wavelengths.
However, when there is astigmatism in the optical system of the spectroscope, the spot elongates in the Y-direction. That is, the spot is widened in the direction perpendicular to the dispersing direction on the light-receiving surface. Therefore, as shown in FIG. 13, an elliptic spot(s) 82 is formed. Note that the spot 82 elongates in the longitudinal direction of the image of the entrance slit in the detector.
A technique for solving this astigmatism problem by correcting the aberration of the spectroscope has been disclosed (Non-patent literature 1). For example, in a spectroscope “SpectraPro series” from Acton Inc., a spectroscope in which a concave mirror has a toroidal surface rather than the spherical surface is used. In this way, the aberration can be corrected. Further, a spectroscope that corrects anastigmatic by using a plurality of concave mirrors has been disclosed (Patent literature 3).
Further, a multifocus confocal Raman microspecroscopy in which a fiber bundle is disposed in front of a spectroscope is disclosed (Non-patent literature 2). In this configuration, a light beam is converted into multi-beams by using a micro-lens array. Then, the multi-beams pass through a lattice-pattern pinhole array. Further, multi-beams emitted from the sample enter the fiber bundle through the lattice-pattern pinhole array. The emission ends of the fiber bundle are arranged in a row. Then, light emitted from the fiber bundle enters the spectroscope.